In a conventional free gas turbine engine, the rotational speed Ng of the high pressure or gas generator spool usually varies in a fixed relationship with the engine power or thrust, as can be seen in FIG. 3. Inlet guide vanes are typically controlled to a predetermined position as a function of the Ng rotational speed. For gas turbine engine applications where the rotational speed Np of the low pressure or power turbine spool is maintained constant (e.g. turbo shafts), a controller usually maintains the Np rotational speed constant through a modulation of the fuel flow and as such reacts to any changes in Np rotational speed due to a change in the load applied to the power turbine. For gas turbine engine application where the rotational speed Np of the low pressure or power turbine spool is changing with thrust demand (e.g. turbofans), the Np rotational speed is controlled at the commanded reference speed through a modulation of fuel flow.
However, improvements are desirable.